1. Field Of The Invention
The present invention relates to a filter element and a filter module comprising the same. More particularly, the present invention is concerned with a filter element comprising hollow fiber filter membranes which are arranged in a lengthwise direction and fusion-bonded together at one or both of their respective end portions to provide a terminal fluid-tight bonding between the filter membranes, and a filter module comprising the same. Because no adhesive is used in the filter element of the present invention for bonding the end portions of the membranes, the filter element can advantageously be used in filtration without the disadvantages which are encountered when filtration is conducted using conventional filter elements in which an adhesive of epoxy resin or urethane resin is used. The disadvantages associated with conventional filter elements occur when the adhesive resin dissolves out into the filtrate. This causes the total organic carbon (hereinafter often referred to as "TOC") in the filtrate to increase and reduce the resistance of the adhesive resin to heat and chemicals so that the fluid-tight bonding between the filter membranes is destroyed.
2. Discussion Of Related Art
Adhesives of epoxy resins or urethane resins have generally been used for bonding the end portions of hollow fiber filter membranes together to prepare a filter element (see, for example, U.S. Pat. No. 3,228,876). However, the use of an adhesive of an epoxy resin or a urethane resin in a filter element is accompanied by the following problems.
In carrying out filtration using a filter element in which an epoxy resin or a urethane resin is used as an adhesive, it is difficult to prevent the dissolution of the adhesive resin from the filter element. Such dissolution of the adhesive resin is undesirable when the filter element is used for preparing ultra pure water.
Hollow fiber filter membranes of fluoro resins which are excellent in resistance to heat and chemicals are available. However, a filter element comprising hollow fiber filter membranes of a fluoro resin having end portions bonded together by means of an adhesive of an epoxy resin or a urethane resin cannot suitably be employed for applications requiring high resistance to heat and chemicals such as, for example, purification of strong acids to be used in producing semiconductors, because the adhesive of epoxy resin or urethane resin is poor in resistance to heat and chemicals. In other words, due to the poor resistance of the adhesive to heat and chemicals, the application of the filter element is limited and, therefore, the excellent properties of the filter membranes of a fluoro resin cannot be realized.
European Patent Application Laid-Open Specification No. 0183256 discloses the use of a fluoro rubber for bonding the end portions of hollow fiber filter membranes together. However, the resistance of commercially available fluoro rubbers to chemicals is still unsatisfactory.
U.S. Pat. No. 3,315,740 discloses a method of bonding tubes together by fusion. Tubes of a thermoplastic material are gathered in a manner such that the end portions of the tubes are in a contacting parallel relationship. The end portions of the gathered tubes is placed within a sleeve having a thermoplastic internal surface and being rigid relative to the tubes. A fluid heated to a temperature at least equal to the softening point of the thermoplastic material, is introduced into the interiors of the end portions of the tubes. Then, a pressure differential is imposed across the walls of the tubes so that the pressure within the tubes is greater than the pressure on the exterior surfaces of the tubes, thereby causing the tubes to be expanded and to be fused with the surfaces of the adjacent tubes.
This method, however, cannot suitably be employed to bond together the end portions of hollow fiber filter membranes (which are porous and, as such, differ from tubes) for the following reasons.
When a heated fluid is introduced into the interior of a hollow fiber filter membrane, not only does the hollow fiber filter membrane shrink but the porous structure of the membrane is also destroyed at the interior portion thereof. As a result, the membrane's effective length for filtration is decreased, thereby rendering the membrane unsuitable for practical use.
A porous hollow fiber filter membrane has a large number of pores that contain air. If a heated liquid is used as the heated fluid and is introduced into the interior of the hollow fiber filter membrane, the air in the pores is squeezed out as the membrane shrinks, thereby causing the hollow fiber filter membrane to have an irregular shape.
Further, porous hollow fiber filter membranes are inferior to tubes in mechanical strength. Therefore, when hollow fiber filter membranes are handled in the same manner as in the above method for bonding tubes, the hollow fiber filter membranes become bent or deformed, thereby causing the porous structures of the membranes to be nonuniform.
As mentioned above, the method of bonding tubes together which is disclosed in U.S. Pat. No. 3,315,740 pertains to a technical field which is entirely different from the technical field to which the present invention belongs, i.e., the field of filter elements each comprising porous hollow fiber filter membranes.
In fact, in attempting to bond the end portions of hollow fiber filter membranes by fusion, various problems have been encountered. The term "fusion" used herein means to bring a material into a viscous state or a state where the solid phase and the liquid phase are in equilibrium.
A hollow fiber filter membrane is a porous body having a porosity of at least 30%, as opposed to a tube and, hence, by the application of heat, a hollow fiber filter membrane shrinks considerably. Therefore, simple fusion of the end portions of hollow fiber filter membranes causes the inner diameters of the end portions of the hollow fiber filter membranes to drastically decrease with great disadvantages. For example, when a hollow fiber filter membrane of an ethylene-tetrafluoroethylene copolymer resin, which has an inner diameter of 0.77 mm, an outer diameter of 1.24 mm and a porosity of 67%, is heated in an oven maintained at 285.degree. C. for 10 seconds, removed from the oven and cooled by air to room temperature, the inner diameter of the hollow fiber filter membrane decreases drastically to 0.30 mm, which is about 39% of the inner diameter before the heat treatment. The outer diameter thereof also decreases drastically to 0.42 mm, which is about 34% of the outer diameter before the heat treatment.
A filter element comprising hollow fiber filter membranes which have undergone such decrease in the inner diameters has severe drawbacks. For example, when carrying out filtration of a liquid containing a large amount of suspended solids by means of such a filter element, the hollow portions of the hollow fiber filter membranes are often clogged with the suspended solids because the inner diameters of the hollow fiber filter membranes are small, thereby causing the filtration to be inoperative. Further, when the filter element is used for the filtration of a highly viscous liquid, because the inner diameters of the hollow fiber filter membranes are small, there is caused a large pressure loss in the lengthwise direction. This leads to a decrease in the filtration pressure in the lengthwise direction, which causes lowering in filtering ability. Therefore, the use of this filter element for such filtration operations is disadvantageous from an economical point of view.
The drastic decrease in the outer diameter of a hollow fiber filter membrane by heat treatment also causes a problem in preparing a filter element from the membranes. Such decrease in the outer diameter makes it difficult to keep the end portions of hollow fiber filter membranes in contact with each other when heating the end portions of the membranes, thereby causing difficulties in effecting bonding of the end portions by fusion.